When to Get Tested?

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

This test measures the amount of urea nitrogen in the blood. Urea is produced in the liver when protein is broken into its component parts (amino acids) and metabolized. This process produces ammonia, which is then converted into the less toxic waste product urea.

Nitrogen is a component of both ammonia and urea. Urea and urea nitrogen are referred to somewhat interchangeably because urea contains nitrogen and because urea/urea nitrogen is the "transport method" used by the body to rid itself of excess nitrogen. Urea is released by the liver into the bloodstream and is carried to the kidneys, where it is filtered out of the blood and excreted in the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood.

Most diseases or conditions that affect the kidneys or liver have the potential to affect the amount of urea present in the blood. If increased amounts of urea are produced by the liver or decreased amounts are excreted by the kidneys, then urea concentrations will rise. If significant liver damage or disease inhibits the production of urea, then BUN concentrations may fall.

What does the test result mean?

Increased BUN levels suggest impaired kidney function. This may be due to acute or chronickidney disease, damage, or failure. It may also be due to a condition that results in decreased blood flow to the kidneys, such as congestive heart failure, shock, stress, recent heart attack, or severe burns, to conditions that cause obstruction of urine flow, or to dehydration.

BUN concentrations may be elevated when there is excessive protein breakdown (catabolism), significantly increased protein in the diet, or gastrointestinal bleeding (because of the proteins present in the blood).

Low BUN levels are not common and are not usually a cause for concern. They may be seen in severe liver disease, malnutrition, and sometimes when a person is overhydrated (too much fluid volume), but the BUN test is not usually used to diagnose or monitor these conditions.

Both decreased and increased BUN concentrations may be seen during a normal pregnancy.

If one kidney is fully functional, BUN concentrations may be normal even when significant dysfunction is present in the other kidney.

Common Questions

1. What other tests are used with BUN to check how my kidneys are functioning?

BUN and creatinine are the primary tests used to check how well the kidneys are able to filter waste products from your blood. Your doctor may also order electrolyte tests, such as sodium and potassium, or calcium to help understand how your kidneys are functioning.

2. How does BUN change with age?

BUN levels increase with age. BUN levels in very young babies are about 2/3 of the levels found in healthy young adults, while levels in adults over 60 years of age are slightly higher than younger adults. Levels are also slightly higher in men than women.

3. What is a BUN/Creatinine ratio?

Occasionally, a doctor will look at the ratio between a person’s BUN and blood creatinine to help them determine what is causing these concentrations to be higher than normal. The ratio of BUN to creatinine is usually between 10:1 and 20:1. An increased ratio may be due to a condition that causes a decrease in the flow of blood to the kidneys, such as congestive heart failure or dehydration. It may also be seen with increased protein, from gastrointestinal bleeding, or increased protein in the diet. The ratio may be decreased with liver disease (due to decrease in the formation of urea) and malnutrition.

Ask a Laboratory Scientist

Form temporarily unavailable

Due to a dramatic increase in the number of questions submitted to the volunteer laboratory scientists who respond to our users, we have had to limit the number of questions that can be submitted each day. Unfortunately, we have reached that limit today and are unable to accept your inquiry now. We understand that your questions are vital to your health and peace of mind, and recommend instead that you speak with your doctor or another healthcare professional. We apologize for this inconvenience.

This was not an easy step for us to take, as the volunteers on the response team are dedicated to the work they do and are often inspired by the help they can provide. We are actively seeking to expand our capability so that we can again accept and answer all user questions. We will accept and respond to the same limited number of questions tomorrow, but expect to resume the service, 24/7, as soon as possible.

Article Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

(Updated 2012 March 23). The Kidneys and How They Work. National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC) [On-line information]. Available online at http://kidney.niddk.nih.gov/kudiseases/pubs/yourkidneys/ through http://kidney.niddk.nih.gov. Accessed November 2012.

August 2007) National Kidney and Urological Disease Information Clearinghouse (NKUDIC). Your Kidneys and How They Work. Available online at http://www.kidney.niddk.nih.gov/kudiseases/pubs/yourkidneys/index.htm#rate through http://www.kidney.niddk.nih.gov. Accessed November 2008.

Proudly sponsored by ...

Learn more about ...

Get the Mobile App

Follow Us

This article was last reviewed on November 29, 2012. | This article was last modified on February 24, 2015.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.